1. Technical Field
The present disclosure generally relates to a polar transmitter and more particularly to a feedforward controlled envelope modulator for a polar transmitter having envelope tracking (ET) or envelope elimination and restoration (EER) function and a feedforward control circuit thereof.
2. Description of Related Art
The communication technology develops very fast, and most people have a handheld communication device in the usual life. The handheld communication device generally comprises a transmitter and a receiver. The transmitter has a power amplifier therein which consumes a great part of total power consumption in transmission link.
Furthermore, regarding a multiple carrier communication system, partial signals have high peak-to-averaged power ratios. Though the multiple carrier communication system provides the high data rate, the required large dynamic rage of power amplifier resulting in low power efficiency.
Due to the low power efficiency of the power amplifier, the lifetime of the battery of the handheld communication device is reduced, and the environment temperature is increased. Since the environment temperature is increased, the problem of bias drift is occurred in the handheld communication device.
Accordingly, a polar power amplifier used in a transmitter is provided to achieve both of the high linearity and the high power efficiency. The polar transmitter can be a polar transmitter having ET function or EER function. Referring to FIG. 1A, FIG. 1A is a block diagram showing a polar transmitter having ET function. The polar power transmitter 10 comprises a power amplifier 102, an envelope amplifier 104, a digital signal processing unit 106, a quadrature upconverter 108, and a transmitting antenna 110.
The in-phase signal I and the quadrature signal Q are received by the digital signal processing circuit 106, and the digital signal processing circuit 106 outputs the amplitude signal A(t), the processed in-phase signal I(t) and the processed quadrature signal Q(t). The quadrature upconverter outputs the converted signal A(t)ej(ωt+φ(t)), wherein the amplitude signal A(t) is expressed as, A(t)=√{square root over (I2(t)+Q2(t))}{square root over (I2(t)+Q2(t))}, ω is the oscillating frequency, t is the variable of time, and the phase signal φ(t) is expressed as φ(t)=tan−1(Q(t)/I(t)).
Referring to FIG. 1B, FIG. 1B is a block diagram showing a polar transmitter having EER function. The polar power transmitter 15 comprises a power amplifier 152 of class E type, an envelope amplifier 154, a digital signal processing unit 156, a quadrature upconverter 158, and a transmitting antenna 160.
The in-phase signal I and the quadrature signal Q are received by the digital signal processing circuit 156, and the digital signal processing circuit 156 outputs the amplitude signal A(t), the processed in-phase phase signal IP(t) and the processed quadrature phase signal QP(t). The quadrature upconverter outputs the converted signal ej(ωt+φ(t)), wherein the amplitude signal A(t) is expressed as, A(t)=√{square root over (I2(t)+Q2(t))}{square root over (I2(t)+Q2(t))}, ω is the oscillating frequency, t is the variable of time, the phase signal φ(t) is expressed as φ=tan−1(Q(t)/I(t)), the in-phase phase signal IP(t) is expressed as IP(t)=cos(φ(t)), and the processed quadrature phase signal QP(t) is expressed as QP=sin(φ(t)).
The polar power amplifier mentioned above is mainly composed of the power amplifier and the envelope amplifier, and the total power efficiency thereof is a product of power efficiencies of the power amplifier and the envelope amplifier. For a long term evolution system, a worldwide interoperability for microwave access system, a wireless local area network system, or a wideband code multiple access system, the envelope amplifier is required to achieve both wideband operation and high power efficiency.
The envelope amplifier can be implemented by a linear regulator, a switching regulator, or a current feedback hybrid envelope modulator. The envelope amplifier implemented by the linear regulator has wideband operation, but low power efficiency. The envelope amplifier implemented by the switching regulator has high power efficiency, but narrowband operation. The envelope amplifier implemented by the current feedback hybrid envelope modulator has medium power efficiency and wideband operation, but the transient response thereof is low.
The envelope amplifier implemented by the current feedback hybrid envelope modulator has a hysteretic current feedback path, and a small current sensing resistor is located in the hysteretic current feedback path. Due to the current feedback control mechanism, the transient response of the envelope amplifier is low, and thus for the wideband or high speed signals, the partial current from the switching regulator will be bypassed to ground through linear amplifier, which results in power inefficiency. Furthermore, the current sensing resistor must be small to reduce power consumption, which is difficulty to be implemented in the integrated circuit design.